This invention relates to the technology of holography and the graphic arts and more specifically to an apparatus and method for making holograms with a reflective or refractive coating. Such holograms are used in a wide variety of security applications, such as on bank notes, credit cards, authenticating devices, I.D. cards (driver""s licenses), product packaging, and the like, as well as for commercial applications such as sales promotion, greeting cards, and the like.
While the discussion herein is directed primarily toward applying holograms to documents or other substrates, the techniques are equally applicable to applying similar devices such as diffraction gratings, multiple diffraction gratings, kinegrams, pixelgrams, and the like. Generally, the techniques described herein apply to the use of any device with a micro xe2x80x9cgroovedxe2x80x9d surface. The micro grooved surface is one containing line (grooves) with a frequency greater than about 6000 lines per inch, although in most cases the frequency may be closer to 25,000 lines per inch. It is the particular pattern of the micro-grooves of a surface that diffracts incident light into an image or other light pattern that is visible to the observer.
Currently, holograms are most commonly mass produced by a series of separate process steps as a thin plastic foil attached to a plastic carrier by a hot melt adhesive layer, then later individually attached to documents or other substrates by a hot stamping process. Such a process is described in U.S. Pat. No. 4,728,377, which is hereby incorporated herein by this reference. Generally, a second hot melt adhesive is included on the surface of the foil, and both adhesive layers are melted as the foil is pressed against the substrate to release the foil from the carrier and attach it to the substrate.
In a common method of mass producing such holograms, a liquid resin (oligomer) is held against the carrier by a surface containing the micro-grooves, which acts as a mold while the resin is hardened by curing. This process transfers the micro-grooves from the master surface to an exposed surface of the cured resin. The micro-grooves are then usually coated with a reflective or refractive material in order to increase the amount of incident light that is reflected from the grooves into a diffracted bean containing an image or other light pattern.
The resin is typically cured photo-chemically by exposure to ultra-violet radiation, and can, alternatively, be cured by electron beam radiation. In the case of an electron beam cure a separate vacuum chamber with an electron gun is used. Furthermore, it can be beneficial to use a vacuum for the coated substrate during the cure to eliminate oxygen from the curing resin and for other beneficial reasons. Subsequent depositing of a metal layer onto the micro-grooves of the cured resin is accomplished separately, typically in a vacuum chamber that is dedicated to that function. Metal is usually applied to the entire resin surface. Portions of the metal layer are sometimes subsequently removed in yet another separate process step with yet different equipment, in cases where this is necessary to provide the desired hologram foil. Some have experimented with applying the metal coating to defined areas by a chemical xe2x80x9creductionxe2x80x9d or electroless chemical process but this has not proved to be particularly satisfactory.
A sputtering process has also been suggested for coating the micro-grooves of the resin with a metal. The sputtering of metal is also performed in a vacuum chamber containing a cathode and anode driven by an electrical power source connected between them. Metal contained on a surface of the cathode is sputtered off of that surface by ion bombardment. The sputtered metal molecules are then driven into the micro-grooved resin surface. The metal thus adheres very well to the micro-grooves.
There has long been a desire to make such holographic material in a continuous process. There has also been a desire to form holograms directly onto the end documents or other substrates in discrete areas thereof. By xe2x80x9cdiscretexe2x80x9d is meant an isolated, defined area on the substrate. Holograms are now applied to substrates in discrete areas by the hot stamping process but it is desired to avoid making the holograms and applying them to substrates by separate discontinuous operations. There has also been a desire to combine such direct hologram formation with printing as part of a continuous printing process. Examples of the foregoing are given in U.S. Pat. Nos. 4,933,120, 5,003,915, 5,083,850 and 5,116,548, which are hereby incorporated herein by this reference.
Briefly and generally, the present invention provides for including two or more processing stations, as well as a substrate supply and take-up mechanism, all in a single vacuum chamber or in a series of two or more adjacent vacuum chambers formed within a common enclosure. When the substrate is in the form of a continuous web of sheet material, for example, the supply is in the form of a roll of the web and the take-up mechanism is a take-up roll. This allows two or more processing steps used to form discrete holograms on the substrate to be performed in-line rather than performing them separately at different times within different vacuum chambers and/or on other equipment. This also allows the processing steps to be performed sequentially without having to transfer the substrate between ambient pressure outside of the vacuum chamber and the reduced pressure within the chamber through air-lock chambers that are inherently lossy.
In one specific example, at least the processing steps of depositing the casting resin on the substrate, molding the resin with a holographic or other micro-groove master, curing the resin while in the mold and coating the cured micro-groove pattern with a layer of material that conforms to the surfaces of the micro-groves to increase their reflectivity in order to efficiently diffract light incident thereon, wherein at least one of these steps must be performed within a vacuum chamber. One such step that needs to be performed within a vacuum chamber is coating of the cured resin micro-grooves by a sputtering or evaporation of a thin metal layer thereon. Another such step that may best be performed within a vacuum chamber is the curing of the molded resin by an electron beam. When the curing and coating steps are carried out within a single vacuum chamber, the additional process steps of depositing and molding the resin on the substrate are most conveniently included as well even though they need not to be performed in a vacuum.
If two or more process steps need to be carried out at different levels of vacuum, two or more vacuum sub-chambers are then included within a primary vacuum chamber the substrate being moved between the two sub-chambers through an air-lock chamber between them that is satisfactory because the pressure difference between the sub-chambers will normally be significantly less than the difference between ambient pressure outside of the vacuum chamber and the pressure within the chamber. One or more other process steps may also be performed in-line on the substrate within the vacuum chamber, such as pre-coating the substrate prior to casting the micro-groove structure thereon, as an early step, removing some of the reflective layer according to a desired pattern and/or coating the completed discrete holograms as a later step. The removal of a portion of the reflective layer, if included in the process, can conveniently be accomplished by an ion etch technique within the vacuum chamber since such a technique needs to be carried out in a vacuum anyway. The substrate may also be subjected to conventional printing within the vacuum chamber in order to maximize the number of operations that may be performed in-line when one of them requires a vacuum.
These techniques can be used to form cast resin micro-groove patterns that are totally surrounded, or substantially surrounded, by the substrate, such as areas in the shape of a circle, rectangle or some irregular shape. These techniques can also be used to form cast resin micro-groove stripes that extend completely along the substrate in the direction of movement though the processing stations with areas of the substrate existing along the sides of the stripes. In a specific example of the use of stripes, multiple parallel stripes are formed along the length of a web and the web is later separated into individual substrates by cutting it with multiple substrates extending across the width of the web so that each substrate has a stripe running along it.
Additional features, objects and advantages of the present invention are included as part of the following description of representative embodiments, which description should be taken in conjunction with the accompanying drawings.